Was America defeated militarily in Vietnam?

Unlike Korea, which is a peninsula, and could be isolated from enemy re-supply; Vietnam was NOT a peninsula and could not be isolated. Therefore, without invading (and ulitimately involving other countries) the US and it's allies could NOT stop the flow of men and material into South Vietnam. This was a scenario far an un-ending war. Consequently, the US and it's allies withdrew from the war, and the enemy proceded south, without the allies to stop them, and conquered South Vietnam. Actually the U.S. was ultimately defeated. They tried every tactic, from massive carpet bombing, to napalm to using agent orange and some DDT. The only thing we didn't use were nuclear weapons. There was no way to win, plus America had very little allies in Vietnam, nothing compared to the Allied turnout in Korea. The nation it self was not defeated, but rather the armed forces. We can call it a "withdrawal" all we want, it was still a defeat.